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Rated current of RCD's

Postby Monkey Magic » Tue May 20, 2008 9:56 am


I notice in my consumer unit the RCD has a rated current of 80a / 30mA - just out of interest how is it decided if its best to use 63a, 80a or 100a rated? Does it depend on the load through the protected mcb circuits or what?

Monkey Magic
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Postby BLAKEY1963 » Tue May 20, 2008 8:22 pm

[quote="Monkey Magic"]Hi,

I notice in my consumer unit the RCD has a rated current of 80a / 30mA - just out of interest how is it decided if its best to use 63a, 80a or 100a rated? Does it depend on the load through the protected mcb circuits or what?


the choice of mainswitch / rcd would depend on the maximum demand of the installation in service.
as from bs 7671 diverscity would b taken into account.

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Postby ericmark » Tue May 20, 2008 10:48 pm

Years ago the supply fuses to most houses were 60 amp and a 63 Amp RCD was ample. The fuse carrier however could accommodate up to 100A fuses and as time has gone on the supply authorities have moved first to 80A and then 100A fuses. Today most consumer units with more than 2 ways are fitted with 100A RCD’s the old 63A RCD’s being mainly used in the garage and shed consumer units feed in turn from a large MCB.
It may be possible to justify fitting smaller RCD’s when considering diversity but unlike the MCB or fuse where if the capacity is exceeded it will trip or blow so fail safe with the RCD overload may cause unseen damage which could adversely effect it’s operation and I have found no documentation allowing diversity to be taken into account when selecting RCD’s which is hardly surprising.
As I have already said 60A and 80A fuses fitted into the same carrier as 100A and since they are normally sealed one has no idea of the size of the fuse inside.
This leaves adding up the values of all MCB’s supplied by the RCD. Two ring mains, a cooker and shower would exceed the 80A so 100A model would be required but where two RCD’s are used One ring main, cooker or shower and one lighting circuit would be within the 80A so the new 17th Edition boards may well use 2 x 80A RCD’s. Also older boards fitted when the smaller mains fuses were the norm may still be fitted with smaller RCD’s.
In your case I see no problem with the existing 80A (This is how much it can handle not a trip figure i.e. you can pass 100A and it will not trip)
As to 30mA this is the largest size for personal protection you can get sockets with 10mA for use outdoors so the socket will trip before main house unit.
When testing 30mA RCD's the testers also check the time. It should not trip at 15mA, It must trip at 30mA in 300ms and at 150mA in 40ms. The yellow test button is only to test the mechanics as if not used for a long time they can stick. But electricians use a special expensive tester to check them with. Other RCD's are used for example where the earthing is not too good you often find 100mA RCD's and for fire protection 300mA RCD's and these may also be S type which means at 5 times the trip value they can take 300ms to trip rather than 40ms there are many specials 500mA versions are used on construction sites for the very big socket outlets for welding sets and the like. There are even auto re-setting versions very expensive and versions where the user can select the amps and the time these are not allowed to be used by general public. There were some old models that worked with voltage these are no longer used but on the odd occasion you come across one.
And last but not least there are now versions that combine the function of an RCD with that of a MCB these are called RCBO's with the new regulations which come into force on the 1st July 2008 we are likely to see a lot more of these. They have a letter B or C in front of the Amps i.e. B16 and the letter shows how quick they will trip with short over current when things start. They will also have a number in a oblong like 10000 which is the max amps they can switch in case of short circuit but to check if this is OK you again need special meters.
You asked out of interest! If it was more than interest please say as there may be special circumstances and I don't want to worry anyone who may think they have the wrong one fitted. There are many things to take into consideration and I have not covered them all so don't worry.

Postby Monkey Magic » Wed May 21, 2008 7:25 pm

Hi Ericmark,

Thanks for the terrific answer, it helped to understand the history of why the different ratings.

I don't doubt my mates abilities but the new cu fitted is a Wylex 17th ed version with twin rcd's, one rated 80a/30mA and the other 63a/30mA which he's used as a matter of course.

Thing is I reckon the 63a probably isnt enough as it looks after upstairs lights 6a, bathroom lights 6a, downstairs ring 32a, kitchen ring 32a and the shower 40a is on there as well, so even allowing for diversity I wasn't too sure.

The 80a looks after immersion heater 16a, downstairs lights 6a, cooker 40a, shower room lights 6a, upstairs new extension ring 32a and then there is the boiler 16a rcbo and 6a rcbo for the smokes on the switch side.

Bearing in mind what you have said, do you think it would be better to change the 63a rcd for a 100a rated? If so does it matter that the 80a rcd would be closer to the DP main switch than the 100a rated?

Wow, I think I've just surprised myself with all that!!

Thanks for your help and knowledge, much appreciated.

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Postby ericmark » Thu May 22, 2008 5:14 am

In real terms I would not think you would have any problems.
Although why put a 63 which is same price as 80 I do not know.
I of course can't say don't change it but it would not worry me if in my consumer unit.

Postby ericmark » Thu May 22, 2008 8:06 pm

Looking at Wylex fully populated from Screwfix we have:-
Dual RCD Split Load Unit
Dual RCD Consumer Unit complete with 10 x MCBs.
• Can Provide 100% RCD Protection
• Circuits on RCD Benefit from DP Protection
• Uses MCBs Throughout
1 x Dual RCD Split Load Unit, 1 x 100A DP Sw, 1 x 80A 30mA RCD, 1 x 63A 30mA RCD, 1 x 40A, 4 x 32A, 2x16A, 3 x 6A SP MCBs.
Assuming a 100 Amp supply
Shower 40 Two ring mains 64
Immersion 16 Spare 16
Lights 6 Lights 6
Cooker + Ring 64 Spare 6
Sub Total A 126 Sub Total B 92 Total 218
Here we can see both the 80A and 63A RCD’s can be overloaded. Although not at the same time. The main point is this whole unit is supplied with the overload ability by the manufacturer. Therefore I can’t see a way I can criticise anyone using the board as supplied by the manufacturer so I must modify my option. And must accept this configuration and assume short length overloads will not affect the long term operation of the RCD’s

Sorry Eric
Last edited by ericmark on Fri May 23, 2008 5:52 pm, edited 1 time in total.

Postby Monkey Magic » Fri May 23, 2008 5:43 pm

Hi Eric,

No need to be sorry at all !! Thanks for checking it out further.

As you say pretty much Wylex are happy to provide you with the 80 & 63a rcd's as part of the kit and also give you enough mcb options to overload both..... :roll:

I would guess I'd be safer getting at least one of the rcd's replaced with a 100a /30mA version maybe both of them??

Kind regards
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Postby ericmark » Tue May 27, 2008 9:48 am

Looking for something else in the regulations I found:-

531.2.8 Where an RCD is used for fault protection with, but separately from, an overcurrent protective device, it shall be verified that the residual current operated device is capable of withstanding, without damage, the thermal and mechanical stresses to which it is likely to be subjected in the case of a fault occurring on the load side of the point at which it is installed.

I would say this about covers it?

All best Eric

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